A tank or similar vessel is often employed at a factory, construction site, warehouse, and the like, to receive, store, and distribute a product. These tanks are adapted to accommodate a variety of different products such as, for example, gasoline, propane, fertilizer, chemicals, fuels, and the like. The tanks that hold these products are often being drained of, and subsequently replenished with, the product. Therefore, sensors are employed to monitor a pressure within the tank so that the amount of product currently in the tank can be determined. One such sensor that has found wide use is a piezo-resistive strain gauge type pressure sensor.
When a piezo-resistive strain gauge type pressure sensor is used to determine the level of fluid in a tank, measurements of the pressure at the bottom of the tank and the temperature of the fluid are taken. The results of these two measurements can be a base for a fluid level, fluid volume, tank inventory control, and additional information. A silicon piezo-resistive pressure sensor can be used for both the fluid temperature and the pressure measurements. Unfortunately, the temperature of the sensor biases both the bridge resistance and the pressure reading. This temperature bias of the bridge resistance allows for a determination of the sensor temperature (and therefore fluid temperature), but the temperature bias must be removed from the pressure calculation.
However, removing the temperature bias is not an easy task. The temperature changes the bridge resistance and bridge pressure sensitivity. This temperature effect is greater when measured across the entire bridge. A three-dimensional graph of pressure, temperature, and bridge voltage creates a surface bended on each end. Previous methods involve using two, multi-order polynomials. One polynomial calculates temperature and the other pressure. This method gives relatively good results for temperature measurements, but it keeps adding the temperature bias to the pressure measurement.
Unfortunately, at least two difficulties exist with conventional pressure transducers (i.e., sensors) used with tanks. First, commercially available pressure sensors adapted for use within tanks are expensive. Therefore, a low cost pressure sensor that can be employed within a tank would be desirable.
Second, the typical pressure sensors all too often include active components. The use of active components in a sensor is, at times, dangerous where, for instance, a tank is holding a product that is flammable. Therefore, a sensor that employs only passive components would be desirable.
The invention provides a low cost, passive sensor and a method of using the same to measure the temperature and pressure of a fluid in a storage tank. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.